1. Field
The invention relates to a surface light source device and its light guide plate.
2. Related Art
In recent years, as the thickness of a mobile device decreases, a surface light source device which is assembled in a mobile device is also requested to become thinner and thinner. To reduce the thickness of the surface light source device, the thickness of the light guide plate has to be decreased. However, even if the thickness of a light guide plate having a flat plate shape is reduced, it is difficult to reduce the height of an LED light source. In the case of using a thin light guide plate having a flat plate shape, the height of the light source becomes larger than the thickness of an end face (light incident end face) of the light guide plate. As a result, a light source disposed so as to be opposed to the light incident end face of the light guide plate is protruded to above the top face of the light guide plate. In the case where the light source is protruded to above the top face of the light guide plate, all of light emitted from the light source does not enter the light incident end face of the light guide plate but a part of the light leaks to the outside of the light guide plate, so that the light use efficiency deteriorates.
To solve the technical problem, there is a case that a light guide plate 13 as illustrated in FIG. 1 is used. The light guide plate 13 has a light introduction part 14 thicker than a light guide plate body 15 at an end of the light guide plate body 15 having a flat plate shape. In the top face of the light introduction part 14, an inclined face 16 which is inclined from a part where the thickness is largest in the light introduction part 14 toward the end of the light guide plate body 15 is formed. A reflection sheet 19 is opposed in the lower face of the light guide plate 13. Since the thickness of the end face (light incident end face) of the light introduction part 14 is equal to or larger than the height of a light source 12, the light guide plate 13 can efficiently take the light of the light source 12 into the light introduction part 14 and guide the light to the thinner light guide plate body 15.
The light which enters the light introduction part 14 from the light incident end face is transmitted to the light guide plate body 15 while being totally reflected between the inclined face 16 and the lower face of the light introduction part 14. The incidence angle of the light propagating in the light guide plate body 15 when the light is incident on the top face of the light guide plate body 15 becomes smaller each time the light is reflected by the inclined face 16. Due to this, the light easily leaks from the top face of the light guide plate body 15 positioned in the proximity of the inclined face 16 to the outside of the light guide plate body 15. In particular, when the inclination angle of the inclined face 16 increases, the light leaks strongly from the top face of the light guide plate body 15 in the proximity of the inclined face 16.
To measure a distribution of light leaked from the top face of the light guide plate 13 having the inclined face 16, as illustrated in FIG. 1, a measuring device Dt is disposed so as to face the top face of the light guide plate 13. FIG. 2 illustrates a distribution of light intensity in a region R1 measured by the measuring device Dt and a distribution of light intensity in a region R2. As illustrated in FIG. 1, light leaked from a flat face part 16a continued to the upper end of the inclined face 16 and light leaked from the inclined face 16 will be examined separately in the top face of the light guide plate body 15. The amount of light leaked from the flat face part 16a is almost zero %. The amount of light leaked from the inclined face 16 is about 54% of the total. The amount of light leaked from the top face of the light guide plate body 15 is about 45% of the total. Therefore, the amount of light leaked from the inclined face 16 is larger than that of light leaked from the light guide plate body 15.
The light leaked from the inclined face 16 is concentrated in a part around the border with the light guide plate body 15 as illustrated in the region R1 in FIG. 2. The light leaked from the light guide plate body 15 is also concentrated in a part around the border with the inclined face 16 as illustrated in the region R2 in FIG. 2. Therefore, the light leaked from the top face of the light guide plate 13 is concentrated in the border part between the inclined face 16 and the light guide plate body 15. The amount of light leaked from the inclined face 16 is larger than that of light leaked from the inclined face 16 as described above. However, as understood from FIG. 2, the light leaked from the light guide plate body 15 is concentrated in a narrow range, and brightness is high, so that the light is conspicuous. Therefore, the light leakage in the region in the proximity of the inclined face, in the top face of the light guide plate body 15 has to be made small.
FIG. 3 illustrates a distribution of light leaked from the light guide plate 13 in a cross section passing the light emission center of the light source 12. The distribution calculated by simulation and it expressed in rays of light. Also from FIG. 3 also expresses that the light leakage is strong on the top face of the light guide plate body 15 near the inclined face.
One of surface light source devices in which light leakage is reduced is, for example, disclosed in JP 5003758 B. FIG. 4 is a perspective view illustrating a surface light source device 11 disclosed in JP 5003758 B. The surface light source device 11 comprises the light source 12 and the light guide plate 13. The light guide plate 13 is formed by integrating the light introduction part 14 having a wedge shape and the light guide plate body 15 having a flat plate shape. The light introduction part 14 has the inclined face 16 in its top face. The inclined face 16 is inclined from the end on the side of the light incident end face toward the end of the light guide plate body 15. In the inclined face 16 of the light introduction part 14, a directivity conversion pattern 17 is provided. The directivity conversion pattern 17 is made by a plurality of V grooves 18 arranged along the width direction of the light guide plate 13. There is a case that the directivity conversion pattern 17 is provided only in a forward region in the light source 12 and there is also a case that the directivity conversion pattern 17 is provided in the entire width of the light guide plate 13.
In the surface light source device 11, light incident on the inclined face 16 is reflected by V grooves 18, so that the incidence angle of light incident on the light guide plate body 15 in the proximity of the inclined face 16 becomes larger. As a result, leakage of light from the light guide plate body 15 is suppressed in the proximity of the inclined face 16.
In the structure such as the surface light source device 11, however, the directivity conversion pattern 17 has to be formed in the inclined face 16, so that a process of a mold or stamper for forming the light guide plate 13 is complicated, and it becomes very difficult to manufacture the mold and the stamper.